After I had created calculator for Humindex - heat index used in Canada, I decided what I will create calculator for Heat Index, which is used in USA, as well.
Below you can find calculators which calculate Heat Index given air temperature in shade and relative humidity. One outputs result in Celsius, for those who used to Celsius degrees, and another outputs result in Fahrenheit, for those who used to Fahrengeit. As usual, all details are given below the calculators.
The Heat Index (or apparent temperature) is based on work by R.G. Steadman published in 1979 under the title "The Assessment of Sultriness, Parts 1 and 2.", so it is the result of extensive biometeorological studies. Originally, results were in tables.
In order to arrive at an equation which uses more conventional independent variables, a multiple regression analysis was performed on the data from Steadman's tables. It was done by Lans P. Rothfusz and described in his work "The Heat Index "Equation" (or, More Than You Ever Wanted to Know About Heat Index) in 1990. You can find it here, for example.
So, the sequence of calculations is:
Heat Index is calculated using this formula:
where T - air temperature in Fahrengeit and RH - relative humidity in percents.
If RH is less than 13% and air temperature is between 80 and 112F, the following adjustment is subtracted from HI:
where ABS - absolute value.
If RH is more than 85% and air temperature is between 80 and 87F, the following adjustment is added to HI:
Usually, HI is calculated using simpler formula (4), then averaged with air temperature. If the result is more than 80F, full formulas should be used.
However, these formulas are not usable for temperature and humidity outside of the range used by Steadman. For temperature it is range from 20 to 50 degrees Celsius. As for humidity, after 30C graph is not linear, so it is better to look it up for yourself, in Steadman, R.G., 1979: The assessment of sultriness. Part I, for example, here.
Source: National Weather Service